Distortion

The EF 50mm f/1.2 USM L shows slight to moderate barrel distortion - a little more than its
smaller cousins.

Note: the chart above has a real-world size of about 120x80cm.

Vignetting

Despite the reduced image circle of the EOS 350D the (full frame) lens exhibits quite pronounced
vignetting (~0.9EV) at wide-open aperture. On full-frame DSLRs the problem should be even more
critical. Nonetheless vignetting problems aren't all that unusual with ultra-large aperture lenses
and on APS-C DSLRs the issue gets drastically reduced when stopping down a little. From f/2 onwards
the issue is basically negligible.

MTF (resolution)

The EF 50mm f/1.2 USM L performed very well in the lab with some of the highest center resolution
figures obtained on the 8mp sensor so far. The center resolution is already very decent at f/1.2 whereas
the corners are less impressive although still "good". Beyond f/2 the center resolution is exceptional
and probably a good deal better than the sensor resolution. The borders improve slowly till reaching
their (excellent) maximum resolution at f/8. While this doesn´t give any reason to complain the
substantially cheaper EF 50mm f/1.4 USM has a slightly better border performance.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Lateral Chromatic Aberrations (CAs)

Surprisingly the EF 50mm f/1.2 USM L is quite prone to lateral chromatic aberrations (color shadows
at harsh contrast transitions). With an average CA pixel width beyond 1.2px the problem is unusually
high for a fix-focal normal lens.

Longitudinal (Axial) Chromatic Aberrations (LoCA)

Similar to other ultra-large aperture lenses the Canon produces a substantial amount of
LoCAs (non-coinciding focal planes of the various colors) at large aperture settings. Here´s an
example at f/2.2: